Exciplex mechanism of the fluorescence quenching in polar media

The formation of exciplexes ( non-emitting or poorly emitting ) is proposed as a cause of the deviations of the experimental data from the classical model of excited state electron transfer in polar solvents yielding radical ion pairs. Experimental evidences for the formation of such exciplexes are discussed. Simulation of the dependencies of the quenching rate constant vs. Gibbs energy of electron transfer according to the exciplex formation model confirms its consistence with the experimental data. INTRODUCTION Excited state electron transfer is commonly supposed to be a general mechanism of fluorescence quenching in the absence of energy transfer and heavy atom effects. Classical kinetic scheme proposed by A.Weller [1,2] assumes that electron transfer step yields contact radical-ion pair which dissociates in polar solvents very fast: k3 0 ki 2 kz 3 M* ,+ Q c--‘(M* Q ) c.-i(Mt Q’) Mt + Q’ (1) hv kf jkd k2 I k3 2 l / . r0 = kf + k, TI i M + Q k = k,,/C 1 + (1+k3z/k30)kz*/k23 I Po/P = T o / T = 1 t K,,[QI = 1 + k,To[Q1 ( 2 ) ( 3 ) ; where v0, P and T ~ , T are fluorescence quantum yields and in the absence and in the presence of a quencher Q respectively. A lot of experimental investigations [3-41 showed the existence of typical dependence of quenching rate constants k, in polar solvents on Gibbs energy of electron transfer AGE,. This dependence has diffusion limit at AGE,<< 0 and thermodynamic limit at AGE,> 0. It is in consistence with the kinetic scheme (l), if one assumes that activation energy of an electron transfer step depends on AGE, according to the empirical Weller equation [1,2] lifetimes